The method generally employed for purifying water, or any other liquid, begins with passage through a first device including filtration and purification means, of the type activated carbon filtering, ion exchange resin filtering or reverse osmosis filtering. At the outlet of the first treatment means, the ultrapure water contains very few ions and is characterized by a resistivity close to, or even equal to, 18.2 MΩ·cm, but still contains organic compounds. It is then passed through a second device in which those organic compounds are oxidized in order to ionize them. During this oxidation the organic compounds are degraded and the atoms of carbon are then present in the form of carbon dioxide gas, which is dissolved in water to form bicarbonate ions HCO3−. This oxidation is obtained by passage in front of an ultraviolet lamp or by adding hydrogen peroxide.
The third purification step consists in polishing the water, i.e. passing it through an ion exchange resin that blocks the ions created during the preceding step and thus completes the purification of the water. During this step the organic compounds that were not degraded during the oxidation phase are not affected.
It remains to determine the purity of the water obtained at the end of this process. One method commonly employed measures its resistivity at the outlet from the oxidation means, which is directly linked to the dissolved carbon dioxide gas content, i.e. to the number of bicarbonate ions, and then determines the resistivity that it would have if the oxidation process had been continued until the end, i.e. if all its organic compounds had been degraded. This process, necessitating an infinite time period, can obviously not be used and it is therefore necessary to employ methods of estimating this limit resistivity.
There is already known, in particular from the Millipore Corporation patent EP0581157, a device and a method for analyzing the purity of water that utilize the difference in the resistivity of the water between the upstream and downstream sides of the oxidation means to estimate this resistivity at infinity. This method uses the device in a first or reference mode to measure the resistivity difference between the upstream side and the downstream side of the oxidation means on water samples that have been exposed for various times to the oxidation means. The exposure times are typically of the order of 10, 20, 30, 40, 50 and 60 seconds. By extrapolation from the curve obtained, it is possible to determine what the resistivity of the water obtained would be after an infinite exposure time, i.e. if all the carbon atoms were to be degraded. Using an appropriate modeling program, such as the MINTEQA2 program described in the publication EPA/600/3-91/021 (1991) of the US Environmental Protection Agency, it is possible to determine from the resistivity at infinity the total organic carbon (TOC) content in the reference water.
A second or purification or analysis mode is then used during which ultrapure water is passed at a given flow rate through oxidation means to determine its content of organic compound impurities and thus to verify that its purity remains nominal. The resistivity difference between the upstream and downstream sides of the oxidation means is measured continuously and the total quantity of organic compounds is deduced therefrom by means of an assumed linear relationship between the resistivity difference measured in the purification mode and the resistivity difference at infinity estimated in the reference mode.